Last week, we — how they were developed, when and where they flew, and ultimately, what killed them. We also enumerated the many challenges facing the future of faster-than-sound passenger travel: noise, fuel consumption, and potentially outrageous per-passenger costs.

Today, we look at the companies promising to bring back supersonic travel. They face immense challenges — like finding engines in a market that doesn’t currently cater to supersonic travel, kickstarting development of renewable aviation fuel, and finding a path to certification with the Federal Aviation Administration, which has never approved a supersonic passenger plane to fly over North America.

With the stakes laid out, it’s time for us to investigate the main players in the supersonic transit (SST) space today — the startups promising to build brand-new passenger planes capable of exceeding the speed of sound. Let’s see just how close they are to bringing air travel back to .

SST Contender #1: EON Aerospace

is the personal project of . EON promises to develop a passenger plane capable of speeds as high as Mach 1.9 (1,254 mph), the NXT-01, aiming to . The NXT-01 concept is a design — which, , means this aircraft would not generate a sonic boom when traveling faster than the speed of sound, allowing it to fly over populated areas at supersonic speeds without disrupting the population below.

EON — whose website has an expired security certificate — offers no real explanation as to how it will solve any of the problems currently facing supersonic travel. The website states that the NXT-01 will feature the “world’s first ever safe-landing mechanism” and that the company is researching the “first-ever bladeless [engine] technology,” with no specifics on how either of these concepts work. I could not find any design specifications or development milestones, such as scale model wind-tunnel testing, for the NXT-01. The company did not respond to my emailed request for comment, and the contact phone number listed on the company’s site was disconnected. No orders appear to have been placed for the NXT-01.

SST Contender #2: Spike Aerospace

Next up is , a supersonic aviation startup , focuses on the niche of private air travel rather than commercial aviation. Its design is known as the S-512, a 12-to-18-seat that promises private-jet amenities with supersonic speed. The startup is aiming for a 2029 release date.

Spike Aerospace, which also did not respond to my request for comment, has very few specifics on its website. It promises the S-512 will offer “proprietary Quiet Supersonic Flight Technology” that will allow it to cruise at Mach 1.6 (1,056 mph) with a ground-level noise . (The Concorde’s , for comparison.) discussing the technology make no effort to explain how it works, beyond “shaping of the wing, fuselage, and tail.” The company apparently flew a dubbed the SX-1.2 in late 2017. At that time, representatives said they expected to have a supersonic piloted test craft flying by late 2019, and the completed . At the time of this writing in late 2022, no piloted test craft appears to have flown, and no supersonic tests have ever been reported.

The startup has a similarly vague approach to engines. , in 2019 Spike Aerospace was in talks with both GE and Rolls-Royce to build engines for its SST; since then, both of those companies have publicly pulled out of the supersonic business, and Spike has not announced a replacement engine manufacturer. In a recent blog post titled “,” Spike Aerospace CEO and founder Max Kachoria admits there is no manufacturer willing to do so at the moment. “Who ultimately supplies the engine will be unveiled in time, as will the successful aircraft company,” he writes.

While details are incredibly vague and the timeline has been moved repeatedly, you can still , if you’re so inclined; it’s only supposed to .

And let’s be real: Even the renders are lazy.

SST Contender #3: Exosonic

, a startup launched in 2019 by a , is taking a slightly different path to supersonic passenger transport by developing a supersonic military drone first. Its supersonic passenger jet concept, however, is promised to be both a low-boom design and capable of Mach 1.9. Exosonic has even partnered with the U.S. Air Force’s to adapt its supersonic jet for use as . Unlike most other companies, however, Exosonic has stated that 2029 is the earliest potential date its SST could hit the market, and the company has shied away from publishing release dates (or offering reservations to purchase supersonic aircraft) in its promotional materials.

So when I asked about the Exosonic SST, a company representative gave me quite an honest answer: the supersonic passenger jet is not the company’s near-term focus. Instead, the company is emphasizing its , which has . The UAV currently in the works is intended to be a low-boom design used primarily for combat training. Future generations of U.S. Air Force pilots could use Exosonic drones as stand-ins for high-speed adversary aircraft.

Since Exosonic’s supersonic drone concept is roughly the size of a fighter jet, it should be easy for the company to power it using any of a number of fighter-jet engines currently on the market. Additionally, since the drone doesn’t require FAA certification for passenger use, as long as the Exosonic drone’s sonic boom is quiet enough to meet noise requirements for aircraft flying over populated areas, it could even enter domestic flight testing, like the Lockheed Martin/NASA X-59 low-boom jet that will soon perform flight tests over the continental U.S. The Exosonic drone design has and begun full-scale development , and Exosonic plans to begin flight testing in the late 2020s.

Exosonic still has plans to develop its passenger airliner, eventually, but the company wants to build on lessons learned with the supersonic drone. An Exosonic spokesperson told me that experience from drone development, support, and maintenance will all be applicable to a future airliner. As for what could power the Exosonic SST, the company spokesperson stated it will use a low- to medium-bypass turbine, but it is “not currently focused on… airliner propulsion solutions,” given the amount of work going into the drone. As a result, it’ll likely be a while before the President — or anyone else — will be flying on an Exosonic passenger jet.

SST Contender #4: Boom Supersonic

Boom Supersonic launched in 2014 after its tech-industry founder and decided to build a supersonic jet of his own. , Boom is the de facto front-runner in the SST startup realm. The company recently finalized its design for a , the Overture, “fares comparable to today’s business class.” Boom has become the focus of lots of media attention in the SST sphere; both and have put down deposits on dozens of Overtures, which are promised to be in revenue service by 2029.

Boom is quite a bit different from the other companies attempting to build an SST. Unlike the Eon NXT-01, the Spike S-512, or the Aerion AS2, the Overture is not intended to be a low-boom design. While this would almost certainly limit it to transoceanic flights, it also makes for a much less complicated airplane to engineer and build. Indeed, the Overture’s current design — a tailless delta wing with four medium-bypass jet engines — was unveiled after and a reported 26 million hours of software simulations, making the concept seem a little more grounded in reality than some other contenders. Even more exciting, Boom has a supersonic single-seat test craft currently in the works, .

Unfortunately, Boom still faces an uphill battle. The XB-1 has been heavily delayed: it was initially supposed to fly in , then , then , then , then . When I asked the company when it plans to have the XB-1 airborne, a representative said the first flight will be next spring.

Despite Boom’s advanced position, the company doesn’t currently have an engine supplier for the Overture. In 2020, Rolls-Royce signed on to build an exclusive engine for the supersonic airliner, but . No other manufacturer has offered to provide Boom with a powerplant. When I asked the company directly about its propulsion plans, a spokesperson noted that a Pratt & Whitney executive had . The company representative added that Boom “remain[s] on track for our engine announcement before the end of the year,” but stopped short of mentioning any specific companies that might be in talks with Boom.

The specific details Boom has publicized have raised further questions about how, exactly, the company plans to dodge . The Overture’s conventional high-boom design means it can never be used in supersonic flight routes over the U.S. or most of Europe. Furthermore, the Overture has an anticipated range of 4,250 nautical miles; , this likely eliminates it from transpacific routes. (Boom Supersonic’s about how quickly a supersonic plane could fly from Los Angeles to Sydney, a route that would require adding 2,000 miles of range to the Overture.)

Unfortunately, even on the transatlantic routes the Overture could complete, the current design only seats a maximum of 88 passengers. Right now, the smallest planes plying the Atlantic routes () have nearly double that capacity.

While the Overture is intended to fly on (SAF) for “net zero emissions,” that still doesn’t change the fact that, with a maximum of 88 passengers aboard, it will be burning vastly more fuel per seat than a comparable subsonic aircraft. When I asked a Boom representative how the Overture could be profitable, the rep claimed that “Overture will cost 75 percent less for airlines to operate than Concorde.” Boom that claim , but has never provided an explanation of how it arrived at that number.

Still, let’s take it at face value. Towards the end of that plane’s service life, British Airway’s fleet of seven Concordes cost the airline roughly a to operate, an inflation-adjusted $2.2 billion. That’s $308 million per plane, per year, for aircraft that spent an average . (The average subsonic commercial airliner .) A Boeing 747, ancient as it is, costs to fly for 450 hours a year; even at double the cost, the thirsty 747 would be an order of magnitude cheaper to operate. British Airways also reported for every hour of flying, despite the extremely low flight-hours the Concordes racked up.

All of this meant the Concorde’s break-even cost per passenger was of the antiquated 747. Even if the Overture is 75 percent more efficient than the Concorde, that still suggests it will be more expensive to operate per customer than even the least efficient quad-engine subsonic jets in the air today. Compared with modern fuel-efficient twin-engine jets like the 787 Dreamliner, the economic case for the Overture is difficult to envision.

The Return of the Concorde?

To say supersonic travel is an uphill battle is an understatement. An SST must battle physics, economics, and governments just to make it out of development. Eventually, those factors grounded the that successfully entered passenger service half a century ago.

Those two SST didn’t last very long faced with the fickle reality of commercial air travel. Can any of the recent crop of hopefuls succeed where past examples failed? That remains to be seen. Let’s hope they can at least find some engines.